Etched sparking plug



Nov. 21, 1939. R. STRIBECK 2,180,779

ETCHED SPARKING PLUG Filed June 18, 1958 Patented Nov. 21, 1939 UNITED STATES 7 ETCHED SPARKING PLUG Richard Stribe'ck, Stuttgart, Germany, assignor to Robert Bosch Gesellschaft mit bes'chrankter Haftung, Stuttgart, Germany Application June is, 1938, Serial No. 214,557

In Germany July 9, 1937 8 Claims. (Cl. 123169) During the operation of a sparking plug in an internal combustion engine, the temperature which the insulator of the plug takes up fluctuates through several hundred degrees Celsius and generally lies between 300 and 900 C. These temperature diiferences occur in that part of the insulator which projects furthest into the combustion chamber and, under certain working conditions of the engine, they take place within a few seconds. The stability of the insulator of the sparking plug to temperature change is not, however, tested only by the temperature variation occurring at a particular determined point of the insulator, but also by the degree of the temperature decrease in the interior and along the surface of the insulator. Especially at the surface of the insulator in the neighbourhood of the points situated at the highest temperature, or on the collar of the insulator, at which the heat is conducted away from the metal body of the plug,

there occur high temperature drops which endanger the stability of the insulator by giving rise to the formation of cracks and flaws.

Hitherto efforts have been made to improve the stability of the sparking plug insulator to temperature change by using special ceramic masses for the manufacture of the said plug, or, by constructional formation of the plug, to cause a decrease of the thermal stresses connected with use. Despite the good result of the steps under-' taken in these directions, the occurrence of cracks in the insulator is still noticed under unfavourable circumstances, especially in plugs which are used in engines which are submitted to sharply changing stresses, and are often submitted to the maximum stress.

It has also been proposed to roughen ceramic insulators or insulators consisting of glass or glazed masses, for the purpose of increasing the resisting'power relative to temperature fluctuations. ening the surface does not increase the stability of sparking plug insulators to temperature change, but that a surface made rough by, for instance, sand jets merely conceals the cracks and flaws which have arisen owing to thermal stress, or makes them less easily visible. Theknown etching of the insulators by means of acids or lyes also does not lead to an increase of the stability to temperature change when the insulators consist of a mass which is attacked throughout by the etching medium, which is the case for instance, in sparking plug insulators made from sillimanite (A12O3.Si02) and steatite insulators.

Now it has been found that such plug insulators However, it has been found that roughr .corundum, spinel, (e. g. MgO.A12O3) or silicon 10 carbide and which, for the remainder, consist of a ceramic mass which is soluble in the etching medium.

The etching medium should be selected to correspond to the particular composition of the insu- 1 lator. Generally, hydrofluoric acid is suitable but salts or lyes, such as sodium carbonate or caustic potash, may also be used in a fused condition as the etching medium.

A considerable improvement in stability to 20.

temperature change is experienced especially by the insulators which contain more than 30% corundum and contain in addition to corundum a burnt ceramic mass serving as the combining agent for the corundum, by etching by means 25 of hydrofluoric acid. Of the components of an insulator of this kind; the corundum is substantially insoluble in hydrofluoric acid, whilethe binding agent is soluble. The etching dissolves the ceramic binding agent of the surface layer and 30 1 leaves on the surface of the insulator corundum crystals which are connected with each other in the manner of a skeleton.

The temperature and concentration of the hydrofluoric acid and the duration of the etch- 35 ing treatment should be adapted to the particular circumstances, especially to the solubility of the component of the plug insulator which is soluble in hydrofluoric acid. Generally, hydrofluoric acid at any temperature up to 100 C. can be used 0 in a concentration of 20-40% H2F2, and the duration of treatment can amount to a few minutes up to two hours. By way of example, the stability to temperature change of an insulator which contains about 70% corundum and the remainder 45 ceramic binding agent, was substantially improved by immersion for a quarter of an hour in hydrofluoric acid containing 40% HzFz and heated to 40 C. The depth of the etching can amount to up to about 0.5 mm.

The process according to the invention is appropriately carried out in such a way that only those parts of the plug insulator which are thermally highly stressed are superficially etched. By means of the drawing, there'is explained by way of example what is to be understood by the thermally highly stressed parts of the plug insulator. In the drawing, there is shown partly in section a sparking plug consisting of the plug insulator a, the lug body b and the middle electrode c. The plug insulator is thermally stressed from the foot :1: to the collar y. At the foot :1: there occur the highest temperatures and greatest temperature fluctuations, and at the collar y, where the heat of the plug insulator is carried off from the plug body I) by means of the packing ring (1, there exists during the operation of the plug, a permanent sharp temperature drop. By etching treatment of those parts at which cracks usually occur, the stability to temperature change can be improved and the formation of cracks can be avoided.

The presence of a glaze makes it more difficult to carry out the process according to the invention, since the glaze layer must be completely dissolved away before the etching medium can act in the desired manner. For this reason it is recommended.to use only such plug insulators as are not glazed at the points to be etched,

' It will generally be appropriate to submit the foot of the plug upwards to the middle of the plug collarin the plug insulator in the drawing, the plug foot x upwards to the dotted line-' to the etching treatment. Especially plug insulators containing corundum in a larger quantity than 30% have proved outstandingly stable to temperature change, when the plug foot, including the lower part of the plug collar, has been superficially etched by means of hydrofluoric acid.

I declare that what I claim is:

1. A process for improving the stability to temperature change of sparking plug insulators consisting of a burnt ceramic mass of insoluble and soluble components, which comprises superficially etching said insulators by contacting them with an etching medium which attacks the soluble components but which does not substantially attack the insoluble components.

2. A process for improving the stability to temperature change of sparking plug insulators consisting of a burnt ceramic mass of insoluble and soluble components, in which the sparking plug insulators are etched superficially by hydrofluoric acid.

3. A process for improving the stability to temperature change of sparking plug insulators con-- soluble components, in which those parts of the plug insulators which are thermally highly stressed are etched superficially by hydrofluoric acid.

5.- A process for improving the stability to temperature change of unglazed sparking plug insulators consisting of a burnt ceramic mass of insoluble and soluble components, which comprises superficially etching said insulators by contacting them with an etching medium which attacks the soluble components but which does not substantially attack the insoluble components.

6. A sparking plug insulator which consists of a burnt ceramic mass containing 30% by weight at least of corundum, in which the foot of the plug insulator has been etched by means of hydrofiuoric acid.

7. A process for improving the stability to temperature change of sparking plug insulators consisting of a burnt mass of insoluble and soluble components comprising about corundum and the'remainder ceramic binding agent which comprises contacting said insulators with hydrofiuoric acid until they are superficially etched.

8. A process for improving the stability to temperature change of sparking plug insulators consisting of a burnt mass of insoluble and soluble components comprising about 70% corundum and the remainder ceramic binding agent which comprises immersing said insulators in hydrofluoric acid containing about 40% HzFz and heated to about 40 C. until they are etched to an amount up to 0.5 mm.

RICHARD STRIBECK. 

